The present invention concerns a process for treating an aqueous solution of titanium sulfate and generally fits into the process for preparing titanium dioxide by the sulfuric method.
The process for preparing titanium dioxide by the sulfuric method consists of treating ilmenite and/or titaniferous slags with concentrated sulfuric acid to obtain a titanium sulfate solution, called black liquor, which is then purified by eliminating the impurities, then hydrolyzed to obtain titanium dioxide.
One of the methods of purifying black liquor is to place the black liquor in contact with an organophosphorus solvent in the presence of acid to selectively extract the titanium sulfate inorganic phase and to thereby separate the impurities from it. The titanium sulfate present in the organic phase is then regenerated into aqueous phase by placement in contact with an aqueous phase.
An aqueous solution of purified titanium sulfate is obtained through this liquid-liquid extraction process. This solution may be hydrolyzed to prepare titanium dioxide. However, this hydrolysis may be conducted only under precise conditions, particularly as concerns the titanium and sulfuric acid contents of the solution to be hydrolyzed.
It is known that the acid factor, defined as the ratio of the concentration of the active acid (free H.sub.2 SO.sub.4 +acid linked to titanium in TiOSO.sub.4 form), expressed in g/l of H.sub.2 SO.sub.4, to the titanium, expressed in g/l of TiO.sub.2, must present a low value, generally below 2.
The value of the acid factor of the final purified titanium sulfate solution may be controlled by adapting the various current flows utilized during the extraction and regeneration steps during purification. This control, however, must be precise, especially when the process for purification by extraction operates in a closed circuit, which is almost always the case.
Furthermore, under some flow conditions which permit the achievement of a good acid factor in the final solution, problems are observed in the extraction unit stages with precipitation of species containing titanium, such as (TiOSO.sub.4, n H.sub.2 O), where n is between 0 and 2, at different degrees of hydration.
And finally, because of its precision, the control used does not allow the value of the acid factor of the final titanium sulfate solution to be modulated at will, thereby affecting the characteristics of the precipitated titanium dioxide.
Therefore, it is generally especially difficult to adjust the acid factor of a titanium sulfate solution during a sulfuric liquid-liquid extraction process, such that the final solution's acid factor is often too high and, if the solution is then hydrolyzed and the hydrolysate then washed and calcined, the pigment obtained is poor.